Asymmetrical RAM works with Flex mode.

Yes. Flex mode is exactly what you need for this. It runs the matching capacity in dual-channel (8GB from the soldered + 8GB from the SODIMM) and the extra 8GB in single-channel. You won’t get full dual-channel performance across all 24GB, but you get more total memory and still a nice boost in bandwidth for the shared portion.

Real-world impact? Unless you run memory-bandwidth-bound tasks (like certain CAD, video encoding, or high-FPS gaming), you probably won’t notice the difference compared to a perfectly matched pair. Most people just see the capacity gain and a snappier system.

This is a totally normal upgrade path for modern laptops. Go ahead.